Air transporting arrangement

ABSTRACT

An arrangement for transporting air with the aid of so-called ion-wind which includes at least one corona electrode and at least one target electrode is shown and described. The corona electrode is a thin wire electrode passing from one side of a duct to another, and the target electrode has a shape such that the distance between the corona electrode and the target electrode is less in the region of the corona electrode ends than in the corona electrode middle. This placement of the electrodes closer together at the ends of the wire electrode provides a uniform corona current over the whole length of the elongated corona electrode which provides the most uniform air flow over the entire cross-sectional area of the duct in which the electrodes are mounted.

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/031,010, filed Jan. 13, 1987, entitled CORONA DISCHARGE AIRTRANSPORTING ARRANGEMENT, now U.S. Pat. No. 4,812,711, issued Mar. 14,1989.

The present invention relates to an arrangement for transporting airwith the aid of a so-called ion wind or corona wind and being of thekind set forth in the pre-characterizing clause of claim 1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

It is known that air can, in principle, be transported with the aid of aso-called electric ion-wind or corona-wind. An electric ion-wind iscreated when a corona electrode and a target electrode are placed at adistance from one another and each connected to a respective terminal ofa d.c. voltage source, the corona electrode and the d.c. voltage sourcebeing such as to cause a corona discharge at the corona electrode. Thiscorona discharge results in ionization of the air, with the air ionshaving the same polarity as the polarity of the corona electrode, andpossibly also in the production of electrically charged aerosols, i.e.air-suspended solid particles or liquid droplets which are electricallycharged as a result of collisions with the electrically charged airions. The air ions move rapidly from the corona electrode to the targetelectrode, under the influence of the electric field, where theyrelinquish their electric charge and return to electrically neutral airmolecules. During their movement from the corona electrode to the targetelectrode, the air ions are in constant collision with the electricallyneutral air molecules, therewith transferring electrostatic forces tothe neutral air molecules, so as to draw these molecules from the coronaelectrode to the target electrode, resulting in the transportation ofair in the form of a so-called ion wind or corona wind.

2. Description of the Related Art

Earlier proposed arrangements for transporting air with the aid ofion-wind are found described, for example, in DE-OS-2854716,DE-OS-2538959, GB-A-2112582, EP-A1-29421, U.S. Pat. No. 3,374,941 andU.S. Pat. No. 4,380,720. These prior art arrangements have been foundextremely ineffective, however, and have not achieved any significancein practice. Air transporting arrangements which utilize the ion-windprinciple and which display marked improvements over the aforesaid knownarrangements, both with regard to efficiency and to practical utility,are described in our international patent application PCT/SE85/00538.

SUMMARY OF THE INVENTION

It has been found, however, that air transporting arrangements of thislatter kind are encumbered with a particular problem, especially whenthere is used an elongated corona electrode, for example an electrodewhich comprises one or more mutually parallel, rectilinear thin wireswhich extend across the airflow path between suitably constructedholders at the ends of the electrode. When using a corona electrode ofthis kind it has been found that the electrode tends to produce a muchhigher corona current per unit of length in the region of its centre,i.e. within the central region of the airflow path, than at its endregions. It would seem that this phenomenon is the result of a screeningeffect created by the fasteners securing the ends of the electrode, andby the wall of the airflow duct normally surrounding the electrodearrangement. In the case of low corona currents, a major part of the twoend portions of the corona electrode may even be "extinguished". Thisphenomenon results in uneven distribution of the ion current over thewhole cross-section of the airflow path and therewith in uneven flowvelocity within said path. In those cases where the airflow passage isdefined by duct walls, those parts of the path cross-section that arelocated axially opposite the two end parts of the corona electrode mayeven have an airflow which is counterdirectional to that desired. Thephenomenon is particularly paramount in the case of airflow ducts whichhave a narrow, elongated rectangular or slit-like cross-section. It willbe understood that this phenomenon greatly impairs the total airthroughput of the arrangement. In an extreme case, the transportation ofair through the duct may cease altogether.

The object of the present invention is to provide an air transportingarrangement of the kind described in the introduction with which theaforementioned problems are no longer found.

This is achieved in accordance with the invention by means of anarrangement of the construction defined in the following claims.

The invention will now be described in more detail with reference to theaccompanying drawings, in which

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 illustrate in perspective and by way of example variousembodiments of the invention in which the target electrode compriseselectrically conductive surfaces located on or adjacent the wall of anairflow duct surrounding the path of air flow;

FIGS. 4-6 illustrate schematically and by way of example furtherembodiments of the invention in which the target electrode comprises anet or grid; and

FIGS. 7-9 illustrate schematically, and by way of example, otherembodiments of the invention in which the target electrode comprises aplurality of mutually parallel, electrode plates or lamellae arrangedperpendicular to the longitudinal axis of the corona electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates schematically an air transporting arrangement whichoperates in accordance with the ion wind principle. The arrangementincludes an airflow duct 1, shown in chain lines, a corona electrode K,and a target electrode M. The electrodes are spaced axially apart withinthe airflow duct 1, with the target electrode M located downstream ofthe corona electrode K, as seen in the desired direction 2 of air flowthrough the duct 1. In the illustrated embodiment the airflow duct 1 hasa narrow, elongated rectangular or slit-like cross-section. The coronaelectrode K comprises a thin, rectilinear wire which extends across theairflow duct 1 along the major axis of the rectangular cross-section ofthe duct, whereas the target electrode M comprises an electricallyconductive surface, or coating which is applied adjacent to or directlyon the inner surface of the duct wall, and which extends fully aroundthe wall. The corona electrode K and the target electrode M are eachconnected to a respective terminal of a d.c. voltage source 3. Thevoltage of this source is such as to create a corona discharge at thecorona electrode K, thereby generating air ions which migrate to thetarget electrode under the influence of the electric field, therebycreating an airflow 2 through the duct 1. With regard to a more detaileddescription of the manner in which the illustrated arrangement operates,the reader is referred to the complete description found in theaforementioned international patent application.

In order to obtain substantially uniform distribution of the coronacurrent along the whole length of the corona electrode K, and therewitha uniform flow of air over the entire cross-section of the airflow duct1, the electrically conductive surface constituting the target electrodeM of the embodiment illustrated in FIG. 1 is formed so as to present ata location axially opposite the end parts of the corona electrode K,surface parts Ma which are situated at a shorter axial distance from thecross-sectional plane that contains the corona electrode K than theparts of the target electrode M located axially opposite the centre partof the corona electrode K. It has been found that the corona current canbe distributed more uniformly along the whole length of the coronaelectrode K, when the target electrode M is constructed in this manner.

With an air transporting arrangement constructed in accordance with theinvention, the target electrode may incorporate a plurality of mutuallyseparated, electrically conductive surfaces or electrode elements thatare connected to mutually different potentials, all having, however, thesame polarity relative the potential of the corona electrode.

FIG. 2 illustrates schematically and by way of example, an embodiment ofthe invention in which the target electrode has the aforedescribed form.As with the embodiment illustrated in FIG. 1, the FIG. 2 embodimentincludes an airflow duct 1 of elongated rectangular or slit-likecross-section, a corona electrode K in the form of a thin, rectilinearwire which extends across the airflow duct 1, and a first targetelectrode M1 in the form of an electrically conductive surface orcoating applied adjacent to or on the inner surface of the wall of theairflow duct 1 such as to extend fully therearound, the corona electrodeK and the target electrode M each being connected to a respectiveterminal of the d.c. source 3. In addition hereto, the FIG. 2 embodimentalso includes a second target electrode M2 which, similar to the firstelectrode M1, comprises an electrically conductive surface or coatingarranged adjacent to or on the inside surface of the wall of the airflowduct 1 and extends fully around the duct, this second target electrodeM2 being connected to the same terminal of the d.c. voltage source 3 asthe first target electrode M1 through a large resistance 4.Consequently, the second electrode M2 will only receive and conduct asmall part of the total ion flow from the corona electrode K and willadjust to a potential which may differ from the potential of the firsttarget electrode M1, but which, of course, has in relation to thepotential of the corona electrode K, the same polarity as the potentialof the first target electrode M1. The two electrodes M1 and M2 can besaid to form together a target electrode arrangement for the ion currentfrom the corona electrode K. The objective of the second targetelectrode M2 and the manner in which it functions are described morefully in the aforementioned international patent application. In theexemplifying embodiment of the invention illustrated in FIG. 2, thefirst target electrode M1 has a uniform axial extension around the fullcircumference of the duct 1. The second target electrode M2, on theother hand, which is located closer to the corona electrode than thefirst electrode M1, has the same form as the target electrode M in theFIG. 1 embodiment. The electrically conductive surface or coatingforming the second electrode M2 thus has parts M2a which extend axiallytowards the corona electrode, or wire K at regions opposite respectiveend parts of the corona electrode, and which are thus located at ashorter axial distance from the cross-sectional plane incorporating thecorona electrode K than the remaining parts of the second targetelectrode M2 located opposite the centre section of the corona electrodeK. As a result, the corona current will be distributed uniformly overthe whole length of the corona electrode K, in the manner desired.

In the case of an air transporting arrangement provided with a secondtarget electrode M2 in the manner illustrated in FIG. 2, the firsttarget electrode M1 may be given the same form as the target electrode Millustrated in FIG. 1, so that both the electrode M2 and the electrodeM1 extend closer to the corona electrode K at regions located axiallyopposite the end parts of said corona electrode.

FIG. 3 illustrates schematically and by way of example a furtherconceivable embodiment of the invention. Similar to the arrangementsillustrated in FIGS. 1 and 2, the arrangements illustrated in FIG. 3includes an airflow duct 1 having a narrow, elongated rectangular orslit-like cross-section, a wire-like corona electrode K which extendsacross the duct 1, and a target electrode M1 in the form of anelectrically conductive surface or coating on the inside of the airflowduct 1, the corona electrode and the target electrode each beingconnected to a respective terminal of the d.c. voltage source 3. Inaddition, two further electrodes M2 are arranged axially oppositerespective end parts of the corona wire K. These further electrodes M2comprise electrically conductive surfaces or coatings located adjacentto or on the inside surface of the wall of the airflow duct 1 and areeach connected to the same terminal of the d.c. voltage source 3 as thetarget electrode M1, through a large resistance 4. These furtherelectrodes M2, which are located solely opposite the two end parts ofthe corona wire K, contribute towards providing a more uniformdistribution of the corona current over the whole length of the coronawire K. The two electrodes M2 may also serve, at the same time, asexcitation electrodes, in a similar manner to that described in theaforementioned international patent application.

FIG. 4 is a schematic, axial sectional view of a further embodiment ofthe invention, in which the target electrode M comprises an electricallyconductive net or grid. In this case, the net or grid is so curved, orarched, that its axial distance to the cross-sectional plane extendingthrough the duct 1 and incorporating the corona electrode K is shorterat those locations opposite the end parts of the corona electrode thanat locations opposite the centre part of said corona electrode. In thisway the magnitude of the corona current is balanced evenly over thewhole length of the corona electrode K, in the manner described.

In order to ensure that the velocity of the airflow is as uniform aspossible over the entire cross-sectional area of the airflow duct, it isimportant not only to distribute the corona current as evenly aspossible over the whole length of the elongated corona electrode, butalso to spread out the ion current from the corona electrode in alateral direction, i.e. in a direction at right angles to thelongitudinal axis of the corona electrode, as evenly as possible overthe airflow duct, i.e. even towards the duct side-walls extendingparallel with the longitudinal extension of the elongated coronaelectrode. This can be achieved with a target electrode of theaforedescribed kind illustrated in FIG. 4 in the manner illustratedschematically in FIG. 5, which is a sectional view of the arrangementaccording to FIG. 4 taken at right angles to the section shown therein.The section in FIG. 5 is thus perpendicular to the elongated coronaelectrode K. As illustrated in FIG. 5, the net or grid target electrodeM is also curved, or arched, so that the axial distance between thecross-sectional plane incorporating the corona electrode K and thetarget electrode M is shorter at the duct side walls extending parallelwith the longitudinal extension of the corona electrode K than withinthe central part of the duct 1. Thus, the net-like or grid-like targetelectrode M has, in principle, the configuration of a double-curve orhemisphere. This ensures better propagation of the ion current from thecorona electrode K over the whole cross-sectional area of the airflowduct 1.

The same result can be achieved when the corona electrode is comprisedof a plurality of mutually parallel, elongated, e.g. wire-like,electrode elements placed side-by-side, in the manner illustratedschematically in FIG. 6, which is a sectional view of the airtransporting arrangement taken at right angles to the section in FIG. 4.As shown in FIG. 6, the target electrode M is, in this case, formed sothat the axial distance between the cross-sectional plane in the duct 1that contains the corona electrode elements K and the target electrode Mis shorter in the region opposite the interspace between said elementsthan in regions opposite thereto. This affords more uniform propagationof the ion current from the corona electrode elements K over the wholecross-sectional area of the duct 1. It will be understood that thenet-like or grid-like target electrode M is, at the same time, curved orarched in the manner illustrated in FIG. 4, so that the axial distancebetween the cross-sectional plane containing the corona electrodeelements K and the target electrode M is shorter at regions opposite theend parts of said elements K than at regions opposite the centreportions thereof.

The target electrode may also have a grid-like configuration whichcomprises mutually intersecting plate-like or lamella-like strips whichextend parallel with the intended direction of air flow, such that thetarget electrode has a substantial extension in said airflow direction.In the case of target electrode of such construction, the side of thegrid facing the corona electrode is formed in the manner aforedescribedwith reference to the illustrations of FIGS. 4-6.

FIG. 7 illustrates schematically a further embodiment of the inventionwhich can be applied to particular advantage in the case of an airtransporting arrangement in which the airflow duct 1 has a broaderrectangular cross-section, or even a square cross-section, and in whichthe corona electrode comprises a plurality of wire-like electrodeelements K arranged in mutually parallel, side-by-side relationship. Inorder to ensure that the corona current is distributed as evently aspossible over the whole length of the wire-like corona electrodes K, andin order to ensure that the velocity of the airflow is as uniform aspossible over the whole cross-sectional area of the airflow duct 1, thetarget electrode M of this embodiment comprises a plurality ofplate-like or lamella-like electrode elements 5 which extend parallelwith one another and also with the direction of the air flow 2, and thelongitudinal extension of which elements 5 is located at right angles tothe longitudinal extension of the wire-like corona electrodes K. Thelamella-like target electrodes 5 are also so arranged that the axialdistance between the plane incorporating the corona electrodes K and theedges of the target electrode elements 5 facing the corona electrodewires K gradually decreases from the target electrode elements 5 locatedopposite the centre parts of the corona electrode wires K to the targetelectrode elements 5 located opposite the end parts of the coronaelectrode wires K. In this way propagation of the corona discharge isachieved right to the ends of the corona electrode wires K, and a moreuniform velocity distribution of the air flow is obtained over the wholecross-sectional area of the airflow duct 1. An advantage is affordedwhen the target electrode M also includes plate-like or lamella-likeelectrode elements 6 which are arranged at the respective ends of thelamella-like target electrode elements 5 and located in the proximity ofand adjacent to a respective opposing wall in the airflow duct 1. Theupstream facing edges of these target electrode elements 6 are therewithadvantageously located at the shorter axial distance from the planecontaining the corona electrode wires K. This also contributes towardsevening out the ion current from the corona electrode wires K in adirection towards the walls of the airflow duct 1, so as to obtain amore uniform flow velocity over the whole cross-sectional area elements6 are particularly valuable when the target electrode elements 5 extendout to an electrically insulated duct wall. In the exemplifyingembodiment illustrated in FIG. 7, the lamella-like target electrodeelements 5 are of varying widths, which does not, however, have anyimportant significance with regard to the function of the targetelectrode in the aforedescribed respects. The important fact is that theedges of the target electrode elements 5 which face the corona electrodewires K are positioned and located in the aforedescribed manner.

In the case of a target electrode constructed in the manner illustratedin principle in FIG. 7, the plate-like or lamella-like target electrodeelements 5 may advantageously have the form illustrated schematically inFIG. 8, which is a sectional view of the air transporting arrangementtaken at right angles to the longitudinal axis of the wire-like coronaelectrode elements K. As illustrated in FIG. 8, the upstream facing edgeof the plate-like or lamella-like target electrode elements 5 directedtowards the cross-sectional plane that contains the corona electrodeelements K is, in this case, profiled in a manner such that the axialdistance between said cross-sectional plane and said edge of the targetelectrode elements is shorter at a location centrally opposite theinterspaces between the corona electrode elements K than opposite saidelements. Similar to the manner described with reference to theembodiment illustrated in FIG. 6, there is also obtained here a moreuniform dispersal of the ion current from the corona electrode elementsK over the whole cross-sectional area of the airflow duct 1.

When the corona electrode comprises solely one single wire-likeelectrode element, the plate-like or lamella-like target electrodeelements 5 are suitably formed in the manner illustrated schematicallyin FIG. 9, which is a sectional view of the air transporting arrangementtaken at right angles to the longitudinal axis of the wire-like coronaelectrode K. The configuration of the edge surfaces of the targetelectrode elements 5 facing the corona electrode K corresponds to theconfiguration of the target electrode described in the aforegoing andillustrated in FIG. 5, and affords an improved and more uniformdistribution of the ion current from the corona electrode K over thewhole cross-sectional area of the airflow duct 1.

An embodiment of the plate-like or lamella-like target electrodeelements 5 according to FIG. 8 or FIG. 9 can be used to advantagedespite the fact that the target electrode has no plate-like electrodeelements 6 located adjacent the duct sidewalls which extend parallelwith the longitudinal extension of the corona electrode K.

It will be evident from the aforegoing that many mutually differentembodiments of the invention are conceivable. In summary it can be saidthat the essential feature of the invention is that the target electrodeis so formed that the axial distance between the cross-sectional planewhich contains the corona electrode and the nearest part of the targetelectrode is shorter at the end parts of the corona electrode than atthe centre regions thereof. Furthermore, when the arrangement comprisesa plurality of mutually parallel elongated corona electrodes the targetelectrode may be so formed that the axial distance between thecross-sectional plane containing the corona electrode and the nearestpart of the target electrode is shorter at the duct sidewalls whichextend parallel with the longitudinal extension of the corona electrode,and also at the region opposite the interspaces between mutuallyadjacent corona electrodes, than at the region opposite the actualelectrode or electrodes.

In the illustrated and described embodiments the corona electrode Kcomprises one or more thin rectilinear wires. It will be understood,however, that the invention can also be applied with other types ofelongated corona electrodes which extend across the airflow path.

Furthermore, the invention has been described and illustrated withreference to an airflow duct or airflow path, of rectangular orslit-like cross-section, since it is with such cross-sectionalconfigurations that the problem concerned is most prevalent. It will beunderstood, however, that the invention can be applied with airflowducts or paths of other cross-sectional shapes, such as circular forinstance, since the problem with which this invention is concerned canalso occur in those cases.

In the aforegoing an air transporting arrangement according to theinvention has been described in detail solely with respect to theconfiguration of the target electrode. With regard to the remainingconstruction of an arrangement according to the invention the reader isreferred to the aforementioned international patent application. Thus,the arrangement need not include a duct which embraces the electrodeswith physical walls. In addition, a suitable screen may be providedupstream of the corona electrode, in order to prevent an ion currentfrom passing upstream from the corona electrode, as described in saidinternational patent application. In all other respects, theconfiguration and positioning of the various electrodes and the voltagesupply thereto may be in accord with the proposals set forth in theaforesaid international patent application.

In those instances in the aforegoing when the target electrode has beenreferred to as comprising electrically conductive surfaces or elements,it should be observed that the current strength of the ion currentpassing from the corona electrode to the target electrode inarrangements of the kind described here is very low, and that the term"electrically conductive" must be understood in relation hereto.

We claim:
 1. An arrangement for generating a flow of air along a flowpath therefor with the aid of an electric ion-wind, comprising at leastone thin wire-like corona electrode having first and second oppositeends attached to support means therefor and extending across said flowpath in a transverse plane substantially perpendicular to the axialextension of the flow path, at least one target electrode located insaid flow path downstream of and spaced from said corona electrode asseen in the axial extension of the flow path and being permeable to aflow of air along said path, and a d.c. voltage source having a firstterminal connected to said corona electrode and a second terminalconnected to said target electrode for creating a corona discharge atsaid corona electrode, said target electrode having such a shape thatthe distance between said transverse plane and said target electrode, asseen in the axial extension of the flow path, is shorter at the ends ofsaid corona electrode than at the center part of the corona electrodelocated between said ends thereof.
 2. An arrangement as claimed in claim1, wherein said flow path is embraced by an air flow duct of rectangularcross-section and having a first pair of mutually parallel oppositewalls which are parallel to the extension of said corona electrode and asecond pair of mutually parallel opposite walls which are perpendicularto the extension of said corona electrode, said target electrode havingsuch a shape that the distance between said transverse plane and saidtarget electrode, as seen in the axial extension of the airflow duct, isshorter in the vicinity of said first pair of airflow duct walls thanalong the central axis of said airflow duct.
 3. An arrangement asclaimed in claim 1, comprising a plurality of mutually parallel thinwire-like corona electrodes arranged side-by-side in said transverseplane, said target electrode having such a shape that the axial distancebetween said transverse plane and said target electrode, as seen in theaxial extension of said flow path is shorter opposite the interspacesbetween mutually adjacent corona electrodes than opposite the coronaelectrodes themselves.
 4. An arrangement as claimed in claim 1, whereinsaid target electrode includes an electrically conductive surfaceextending parallel to the axial extension of said flow path andsurrounding the same, said electrically conductive surface having formedthereon at locations opposite said ends of said corona electrode, asseen in the axial extension of the flow path, portions projectingtowards said corona electrode and extending closer to said transverseplane than the remaining portion of said electrically conductive surfaceforming said target electrode.
 5. An arrangement as claimed in claim 1,wherein said target electrode comprises a first electrically conductivesurface extending parallel to the axial extension of said flow path andsurrounding the same and also two further electrical conductive surfaceslocated, as seen in the axial extension of the flow path, opposite arespective one of said ends of said corona electrode and between saidends and said first electrically conductive surface, said furtherelectrically conductive surfaces being parallel with the axial extensionof the flow path and embracing solely those parts of said flow path thatlie downstream of said ends of said corona discharge.
 6. An arrangementas claimed in claim 5, wherein said further electrically conductivesurfaces are connected to said second terminal of the d.c. voltagesource through a large resistance.
 7. An arrangement as claimed in claim4, wherein said flow path is embrace by an airflow duct having wallsextending in the axial extension of the flow path, said electricallyconductive surfaces of the target electrode being arranged adjacent toor on the walls of said flow duct.
 8. An arrangement as claimed in claim1, wherein said target electrode comprises a net extending across saidflow path and being so curved as to fulfill said conditions regardingthe distance between said transverse plane and the target electrode. 9.An arrangement as claimed in claim 1, wherein said target electrodecomprises a grid-like structure extending across said flow path ancomposed of elongated lamella-like mutually intersecting strips arrangedwith their side surfaces parallel with the axial extension of the flowpath, the side of said grid-like structure facing said corona electrodebeing so constructed as to fulfil said conditions regarding the distancebetween said transverse plane and the target electrode.
 10. Anarrangement as claimed in claim 1, wherein said target electrodecomprises a plurality of elongated lamella-like electrode stripsextending mutually parallel across said flow path in a directionperpendicular to the direction of extension of said corona electrode,each of said electrode strips having a first longitudinal edge facingupstream in said flow path and a second longitudinal edge facingdownstream in the flow path and side surfaces between said longitudinaledges which side surfaces are parallel with the axial extension of theflow path, said electrode strip being so arranged that the distancesbetween said transverse plane and said upstream facing edges of theelectrode strips, as seen in the axial extension of the flow path,decreases gradually from the electrode strips located opposite a centralpart of said electrode to the electrode strips located opposite saidends of the corona discharge.
 11. An arrangement as claimed in claim 10,wherein said upstream facing edge of said electrode strip is so formedthat the distance between said transverse plane and said upstream facingedge, as seen in the axial extension of the flow path, is shorter at theends of the electrode strip than at the center part thereof.
 12. Anarrangement as claimed in claim 10, comprising a plurality of thinwire-like corona electrodes arranged mutually parallel and mutuallyspaced in said transverse plane, said upstream facing edge of each ofsaid target electrode strips being so formed that the distance betweensaid transverse plane and said upstream facing edge, as seen in theaxial extension of the flow path, is shorter opposite the interspacesbetween mutually adjacent corona electrodes than opposite the actualcorona electrodes themselves.
 13. An arrangement as claimed in claim 10,wherein said target electrode comprises two additional elongatedlamella-like electrode strips arranged at the opposite ends of saidfirst mentioned target electrode strips and extending perpendicular tothese.